The avoidance of mode locking in laser gyroscopes through the use of mechanically vibrated or individually dithered mirrors is known in the art. Mode locking is the tendency of two counter rotating beams of ring laser radiation (hereinafter called "light") to appear to have the same frequency at low levels of angular velocity due to noise or light scattering, thus causing a loss of the desired beat frequency. It is also known in the art to dither or rotate the entire laser gyroscope in order to prevent mode locking. Dithering of the mirror or the entire gyro causes vibrations which, if transmitted to the case, can interfere with the input or output of other instruments or gyros within a navigational system in which the first gyro is utilized. Elimination of this vibration problem is accomplished by the utilizion of a isolation arrangement within each gyro. An example of a isolation system which reduces the vibrational energy of an oscillating gyro may be found in U.S. Pat. No. 4,115,004 by Hutchings et al. which issued on Sept. 19, 1978 and is assigned to the assignee of the present invention.
The isolation system shown within the Hutchings et al. '004 patent utilizes a dual spring system which mounts a counterweight for isolating the oscillatory motion of the gyro. This spring system includes a first set of springs mounted between the frame or case and the gyro and a second set of springs mounted between a case and the counterweight. The dual spring system of Hutchings et al. '004 works well under some conditions but is not a practical system under all operating conditions, such as temperature variations, due to the need to match the springs precisely in each system. The dual system can be fine tuned in the laboratory; but, as temperatures vary, the tuning is lost causing an increased amount of vibrational energy to be transmitted to the case of the gyro. One reason it is difficult to tune a dual spring system is because the gyro case must be attach precisely at the node between the two spring systems. The present invention reduces this problem by attaching the node to the case through a third spring system wherein the point of attachment need not be as accurate as in a dual spring system.
A prior art example of a single spring system used to isolate vibrational energy from an aircraft instrument is shown in U.S. Pat. No. 3,464,657 by Bullard which issued on Sept. 2, 1969 and is assigned to the United States of America. In this system, the single set of springs is connected between the frame and the mounting platform which, in the present invention, is the equivalent of the gyro.
While the preferred embodiment described hereinbelow describes a ring laser gyro utilizing a rectangular laser path, it will be understood that gyros utilizing triangular laser paths are also known and that the present invention is not intended to be limited by the description of a rectangular path. A ring gyro laser utilizing a triangular laser path is described in U.S. Pat. No. 4,152,072 by Hutchings which issued on May 1, 1979 and is also assigned to the assignee of the present invention.
For a more basic discussion of the operation of ring laser gyros, attention is directed to a text book entitled Laser Applications, edited by Monte Ross, Academic Press, Inc., New York, New York 1971.